The present invention generally relates to lighting systems and related technologies. More particularly, this invention relates to fluorescent lamps and coating systems utilized by fluorescent lamps to generate visible light.
Fluorescent lamps have been in use and commercialization since the 1930s. More recently, both consumers and producers have voiced increased concerns for energy efficiency and environmental impact of products, spanning all industries, including the lighting industry. As such, fluorescent lamps have seen an increase in usage due to their increased energy efficiency as compared to conventional incandescent lights. Fluorescent lamps see a great deal of competition from light-emitting diode (LED) lights due to a potential for greater efficiency and luminosity of LEDs. Significant effort and research have been made in the interest of improving fluorescent light lumen output without increasing power requirements or significantly increasing material costs.
A nonlimiting example of a fluorescent lamp 10 is schematically represented in FIG. 1. The lamp 10 is represented as having a sealed glass tube comprising of a glass envelope or shell 12 enclosing an interior chamber 14. The chamber 14 is preferably at very low pressure, for example, around 0.3% atmospheric pressure, and contains a gas mixture having at least one constituent that can be ionized to generate radiation that includes ultraviolet (UV) wavelengths. According to the current state of the art, such a gas mixture includes one or more inert gases (for example, argon) or a mixture of one or more inert gases and other gases at a low pressure, along with a small quantity of mercury vapor. Electrodes 16 inside the chamber 14 are electrically connected to electrical contact pins 18 that extend from oppositely-disposed bases 20 of the lamp 10. When the contact pins 18 are connected to a power source, the applied voltage causes current to flow through the electrodes 16 and electrons to migrate from one electrode 16 to the other electrode 16 at the other end of the chamber 14. In the process, this energy converts a small amount of the liquid mercury from the liquid state to a charged (ionized) gaseous (vapor) state. The electrons and charged gas molecules move through the chamber 14, occasionally colliding with and exciting the gaseous mercury molecules, raising the energy level of the electrons in the mercury atoms. In order to return to their original energy level, the electrons release photons.
Due to the arrangement of electrons in mercury atoms, most of the photons released by these electrons are in the ultraviolet (UV) wavelengths. This is not visible light, and as such for the lamp 10 to emit visible light these photons must be converted to a visible light wavelength. Such a conversion can be performed by a coating 22 disposed at the interior surface of the glass shell 12. The coating 22 comprises phosphor powders and, as represented in FIG. 1, is separated from the glass shell 12 by a UV-reflecting barrier layer 24 of, for example, alumina (Al2O3). As known in the art, the coating 22 can be produced by applying to the shell 12 a suspension containing particles of the desired phosphor(s) combined with one or more surfactants, dispersants, thickening agents, etc., and then performing a lehring operation that involves heating the applied suspension to remove suspension components, leaving the phosphor particles (and potentially other particle materials) to form the coating 22 on the shell 12. The UV wavelengths emitted by the ionized mercury vapor are absorbed by the phosphor composition within the coating 22, resulting in excitation of the phosphor composition to produce visible light that is emitted through the glass shell 12. More particularly, when electrons of the phosphor atoms are struck by photons, the electrons become excited to a higher energy level and emit a photon to return to their original energy level. The emitted photon has less energy than the impinging photon and is in the visible light spectrum to provide the lighting function of the lamp 10. The color and luminosity of the lamp 10 are largely the result of the phosphor or phosphors used in the coating 22.
The mercury in low pressure fluorescent lamps predominantly emits UV radiation having a wavelength of 254 nm, and to a lesser extent a wavelength of 185 nm. As used herein, “predominantly” and “predominant” mean that something contains more of one constituent (the “predominant constituent”), e.g., by weight, volume, molar, or other quantitative percent, than any other individual constituent. As these terms are used herein in relation to radiation, “predominantly” and “predominant” signify a wavelength that is more prevalent in a band of radiation than any other individual wavelength. Some estimates are that roughly 90% of UV radiation generated by low pressure fluorescent lamps is at the predominant 254 nm wavelength, with the balance (roughly 10%) being the 185 nm wavelength. Both of these wavelengths fall within a wavelength range known as ultraviolet subtype C. Phosphors used in low pressure mercury lamps are typically excited by different ranges of wavelengths that encompass the primary wavelength (254 nm) to absorb as much UV radiation as possible. The efficiency and effectiveness of fluorescent lamps and their coating systems can differ based on what phosphors are used and what wavelengths of light are absorbed.
The apparent, or perceived, color of a light source can be described in terms of color temperature, which is the temperature of a black body that emits radiation of about the same chromaticity as the radiation considered. A light source having a color temperature of 3000K has a larger red component than a light source having a color temperature of 4100K. As additional examples, a fluorescent lamp having a perceived “warm white” color may have a correlated color temperature (CCT) of approximately 3000K, whereas a fluorescent lamp having a perceived “cool white” color may have a CCT of approximately 4000K. Another measure of fluorescent lamp performance is the color rendering index (CRI). The CRI of a light source does not indicate the apparent color of the light source, but instead is a quantitative measure of the ability of a light source to reproduce the colors of objects faithfully in comparison with an ideal or natural light source. CRIs can only be accurately compared among two light sources having the same CCT. The highest possible numeric CRI value is 100. Incandescent lamps, which are essentially blackbodies, have CRIs of 100. Typical LEDs have CRIs of 80 or more, with CRIs of up to 98 being claimed, whereas fluorescent lamps typically have CRIs in a range of about 50 to about 90. In this regard, a high CRI for fluorescent lamps can be considered to be about 80 and higher, particularly at least 87.
Another metric by which fluorescent lamp performance can be gauged is light output or lumen maintenance, which characterizes the ability of a lamp to provide roughly the same amount of luminosity over its life span. All lamps exhibit some reduction in luminosity over time, though some more so than others, depending on the phosphors they utilize. Zinc silicate phosphors such as manganese-doped zinc silicate green phosphor (ZSM) are particular but nonlimiting examples of phosphors that can exhibit poor lumen maintenance characteristics, with other notable examples including strontium-based phosphors such as tin-doped strontium phosphate red (SR) and tin-doped strontium phosphate blue phosphor (SB), and typically to a lesser extent halophosphors. ZSM phosphor has been used separately in lamps that emit green light and in combinations with other phosphors to emit white light. As nonlimiting examples of the latter, phosphor blends containing ZSM, SR and SB have been used or considered for use in high CRI lamps formulated for color temperatures of about 4100K, and phosphor blends containing ZSM, cerium magnesium borate (CBM), europium-doped strontium aluminate (SAE), and halophosphors have been used or considered for use in high CRI lamps formulated for color temperatures ranging from 2700K to 3500K. Though these phosphor blends have certain desirable qualities, for example, excellent color rendering, initial color properties, and/or initial light levels, they suffer from poor lumen maintenance characteristics, attributable at least in part to their ZSM content.
Poor lumen maintenance is characterized by a rapid depreciation of a phosphor during normal operation of a lamp, and can be particularly evident in highly loaded lamps such as T5, T5HO, CFL, and BIAX types. The poor lumen maintenance characteristics of ZSM, SR and SB may be attributed in part to their propensity for mercury consumption (binding), and the poor lumen maintenance characteristics of ZSM can be further attributed at least in part to sensitivity to 185 nm radiation emitted by low pressure mercury lamps. Various attempts have been made to improve lumen maintenance in lamps that contain ZSM and/or other phosphors that exhibit poor lumen maintenance characteristics. As examples, chemical vapor deposition (CVD) coatings and surface washes have been attempted, but with limited success. In addition, barrier coatings and additions of alumina or silica have been investigated, but have not been entirely successful.
In view of the above, it would be desirable to improve the lumen maintenance characteristics of fluorescent lamps that contain certain phosphors prone to poor lumen maintenance, including but not limited to ZSM, SR, and SB.